Laser beam machining of a workpiece by two-photon processes usually occurs by shifting a laser beam focus over or in the workpiece and is also known as laser writing or laser scanning lithography. Apparatuses and methods for this purpose are known from the state of the art and usually utilize the polymerization of a workpiece, e.g. of a photosensitive resist. A respective description can be found in J.-M. Lourtioz, Nature Materials 3, 427 (2004) and M. Deubel et al., Nature Materials 3, 444 (2004). A microscope for laser beam writing has been described for example in U.S. Pat. No. 5,034,613. Materials suitable for laser beam writing have been explained for example in the publication Miller J. et al., “Laser-Scanning Lithography (LSL) for the Soft Lithographic Patterning of Cell-Adhesive Self-Assembled Monolayers”, in Biotechnology and Bioengineering, Vol. 93, No. 6, Apr. 20, 2006, pages 1060-1068.
The goal thereby is to achieve a very high resolution, which is why objectives of high numerical aperture are used in order to advance into a range of less than 100 nm with the machining precision. Generally, oil immersion lenses having a numerical aperture of approx. 1.4 are used. The structures to be written into the workpiece themselves usually extend a few 100 μm in all directions of space. Photosensitive lacquers (resists) are used among others as suitable workpiece materials which resists are spin-coated on a cover glass.
The reproduction of the short-range order and long-range order and a precise reference to at least one boundary surface are relevant for the quality of the structuring, in addition to the resolution. For scanning, which means for shifting the focus position in or on the workpiece, mostly highly precise piezo tables are therefore used for moving the workpiece. It needs to be ensured however that the position of the sample remains as stable as possible during a possibly long writing/exposure process both transversally to the optical axis (therefore laterally) and longitudinally thereto (therefore axially). This is essential for machining the workpiece in certain production processes, as also the absolute referencing of the focus position.
The problem arises however that the referencing of the absolute position of the focus is often very difficult or even impossible, and can especially change during the machining process.
It is further known in the state of the art to use triangulation methods, imaging methods with contrast evaluation and the determination of positions by means of obliquely positioned confocal slit diaphragm for autofocus functions. In the case of triangulation methods, a collimated laser beam is reflected into the pupil plane of a lens and conclusions are drawn on the z-position of the laser light reflected from the sample from the progression of this laser beam relative to the imaging beam path. In the case of conventional sizes of workpieces which are machined with laser scanning lithography, the autofocus quality of such systems would be insufficient. Moreover, fluctuations can be determined as to whether the result of the measurement is made at the center or the edge of the workpiece or of the detector employed for this purpose. A triangulation method is therefore usually performed iteratively, which is relatively time-consuming.
In the case of imaging methods with contrast evaluation, a sample is illuminated with a specific intensity distribution, generally in that a grating is placed in a field stop plane of an illumination beam path. A series of pictures is taken with different distances between imaging optics and sample and the picture with the highest contrast in this series, is determined to which picture the optimal focal distance is assigned. Examples for an autofocus device by means of contrast analysis of a pattern projected to a sample can be found in U.S. Pat. No. 5,604,344 or U.S. Pat. No. 6,545,756.
The fact that workpieces machined by means of two-photon processes are usually transparent represents a problem here because there are no structures in the workpiece as a result of its transparency.
It is further known from DE 10319182 A1 for example to provide the determination of position by means of an obliquely positioned confocal slit diaphragm, in that a slit diaphragm is positioned in a field stop plane of the illumination beam path and is projected to a sample. The light reflected from the sample is directed to a CCD line which is arranged in an inclined manner relative to the slit diaphragm and the position on the CCD line is determined where the reflected light has a maximum. This method is very quick, but has problems with impurities on the sample or the sample surface which can lead to fluctuations in intensity. Moreover, it is necessary to apply a high amount of adjustment in the projection of the gap onto the CCD line, because the gap needs to be very narrow in order to enable the achievement of high precision. In particular, the slit diaphragm is effective at the edge of the picture field of the lens, which considerably limits the precision. This approach can therefore not be used for laser scanning lithography.
All methods have in common that they are capable of finding the focal plane very precisely, but are able to determine the position of this focal plane within the sample in a very limited way, especially concerning further boundary surfaces.
The use of several autofocus beam paths is described for example in WO 00/43820.